A number of safety devices of this general type are available, such as that described in U.S. Pat. No. 4,714,915 (Hascal et al) issued Dec. 22, 1987. Such a device may be installed on a safety helmet worn by the user. However, such devices pose significant disadvantages in practise.
These devices employ the principle of electrostatic induction on parallel plates, in which one plate is an inductor plate, or antenna, and a floating reference voltage is produced in a parallel plate. The antenna is connected to the input of a detector circuit which derives a DC signal corresponding to the intensity of the electrostatic field at the position of the antenna. A comparator compares the DC signal with a selected reference signal, and produces an output signal when the derived DC signal exceeds the reference signal, sounding an audible alarm.
A main disadvantage to this system is that the antenna, typically comprising a stripline antenna mounted on or around the user's helmet, can be shielded from the electrostatic field by the user. Thus, depending upon the position and orientation of the user relative to the energized conductor, the electrostatic field intensity at the position of the user may exceed the selected limit without sounding the alarm, if any portion of the user's body is interposed between the field source and the antenna. On the other hand, in certain orientations the alarm will sound when the user himself is not within the selected minimum distance of the high voltage conductor, as determined by the selected maximum intensity of the electrostatic field, if any part of the antenna strays within that distance. The latter situation can be an annoyance, while the former situation can be dangerous.
The present invention overcomes this disadvantage by providing an electrical contact avoidance system in which the body of the user forms the antenna. Since it is ultimately the user, and not the device, which must avoid contact with the high voltage conductor, by utilizing the user's body as the antenna the alarm will not sound unless part of the user's body approaches the field source too closely; nor can the user stray within the minimum selected distance from the field source without triggering the alarm. The electrical contact avoidance device of the present invention thus avoids the annoyance of false alarms and at the same time ensures that whenever any part of the user strays within the selected minimum distance from the conductor the alarm will sound, regardless of his or her orientation.
The system of the present invention further provides a support basket for a powered aerial access device which may be coupled to ground or to a high voltage conductor being serviced, or allowed to assume a floating voltage reference, a platform insulated from the basket to which the lineman is directly or capacitively coupled, and a monitoring circuit connected between the platform and the basket. Where the basket is grounded, or electrically isolated from the conductor, the monitoring device will sound an alarm if the lineman approaches a high voltage conductor too closely. Alternatively, the basket and lineman may be coupled or "bonded" to the high voltage conductor, enabling the lineman to work hands-on with the conductor, and the alarm will sound if the lineman approaches a grounded conductor or a different phase or voltage conductor. In either case, the lineman is protected from electrical shock by avoidance of any conductor energized to a voltage or phase different from that of the lineman's body.
The subject invention is based on the premise that an electrostatic field produced by a high voltage conductor induces a detectable voltage in a person's body, as in any other conductor. This induced voltage is directly related to the intensity of the electrostatic field, and thus the proximity of the person to the field source, i.e. the high voltage conductor.